The present invention relates generally to the recovery of metal values from liquid solutions by employing electrolytic metal separation apparatus, and more particularly to an improved metal recovery system for effecting such electrolytic separation and the recovery of the separated metal values.
The electrolytic recovery of ionized metal values from metal-containing liquid solutions such as silver in photographic and radiographic film processing solutions has been achieved in metal recovery apparatus using well known electrolytic separation practices. In such recovery practices an electric current flowing between an anode and cathode through a metal-laden solution ionizes metal values in the solution and effects deposition of the metal ions in the solution onto the cathode for subsequent recovery.
A type of metal recovery system which has been found to be successful utilizes a cathode in a cylindrical form with an elongated anode disposed within the cathode along the longitudinal axis thereof. A metal-containing solution is circulated through the chamber defined between the anode and cathode while an electric current is established between the anode and cathode through the solution to effect the deposition ionized metal values in the solution onto the cathode. One such system of this type is described in patentee's U.S. Pat. No. 4,026,784 which issued May 31, 1977. In this patented system the metal-laden liquid solution is circulated by an external pump throughout the vertically oriented chamber between the cathode and anode along a generally helical flow path adjacent to the vertical cathode walls. This helical flow pattern exposes metal ions in the liquid solution to a substantial area of the cathode walls to thereby provide an efficient metal recovery operation. This patented system also utilizes a current control mechanism for providing preselected current densities at the cathode for enhancing the rate of metal recovery as well as increasing the quantity of metal recoverable from the solution. For example, in a silver recovery operation, a high cathode current density is used to provide a maximum deposition rate for the metal into the cathode while sufficient silver ions are in the solution to prevent the occurrence of undesirable sulfiding. However, when the liquid solutions become sufficiently depleted of silver so as to contain an insufficient concentration of silver to inhibit sulfiding, a lower cathode density is then used to provide additional silver recovery. In as much as features in this patented system correspond generally to or are similar to features useful in the practive of the present invention, the aforementioned patent is incorporated herein by reference.
While metal recovery systems, such as described in the aforementioned patent, provide satisfactory levels of metal recovery from metal-containing solutions such as silver from photographic and radiographic film processing solutions, there were found to be some shortcomings or drawbacks which detracted from the overall efficiency and desirability of these previously known systems. For example, when utilizing a helical flow pattern for the metal containing solution, it was discovered that vortices were formed in the circulating solution at locations in the chamber intermediate the anode and the cathode with such vortices producing dead spots in the solution with zero rotational direction. With these vortices introduced into the circulating solution, a significant amount of the metal contained in the solution tends to precipitate out of the solution and collect in the center of the vortices so as to substantially reduce the percentage of metal in the solution that is collectable at the cathode. Another shortcoming found to be attendant with such previously known metal recovery systems is that the removal of the accumulated metal on the cathode often involved considerable disassembly of the system so as to result in a substantial expenditure of labor and downtime for effecting the actual recovery of the metal removed from the metal-containing solution.